Various power systems, such as crane applications aboard vessels or in harbors, comprise a plurality of electric loads supplied from the same power source. FIG. 1 shows an example of such a power system comprising three electric loads 21, 22, 23. The electric loads 21, 22, 23 in this example are electric drives, each comprising an electric motor driven by an inverter or a frequency converter. The electric loads are each connected to the same power source 10, such as a supply line or network, which may be an AC or DC power source, depending on the application in question.
Sometimes the available maximum power from the power source in such power systems comprising several loads is temporarily or constantly limited in comparison to the total power required when all the electric loads, such as motions of a crane, are simultaneously in use with full power. In the example of FIG. 1, the individual maximum powers required by the electric loads 21, 22 and 23 could be, for instance, 20 kW, 10 kW and 5 kW, respectively, and the maximum power capacity of the power source could be only 30 kW, which is less than the total maximum power of 35 kW of the loads. In this kind of situation the power source, a feeder network for example, cannot feed all the loads with full power. This kind of situation may be due to an underpowered power source (possibly an intentionally limited maximum power) or a temporary problem in the power source, for example. In the case of a crane application, for instance, the power required by the system is related to the speed range used in the crane motions according to the following equation:P=Q*V*g/efficiency,  (1)
where:
P=motor power required in kW
Q=load in tons (1000 kg)
V=speed in m/sec
g=standard acceleration of gravity (≈9.81 m/s2)
In the case of a crane application, for instance, this means that the crane operator needs to select manually one crane motion at a time or to drive slowly to reduce the total power required by the crane, which may be problematic.
U.S. Pat. No. 4,117,537 discloses a solution for limiting energy consumption. In the disclosed solution, when a desired power consumption rate is exceeded, the loading of modulating loads is reduced. If more than one modulating load is present, a priority system of modulating load reduction can be accomplished by setting different reduction rates for different loads.
A drawback related to the above solution is that the loading of all the modulating loads is reduced at the same time, although possibly at a different rate.